Many medium access control protocols for wireless sensor networks have been proposed for allowing low power transmissions in these networks whose nodes are generally energy-restricted, due to the fact that they are often battery powered. Actually, in these networks, nodes periodically go to sleep and wake-up using the same schedule. Then, when a node wants to transmit a packet, it knows when all the network will be awaken and can transmit at this time. Such a method requires perfect synchronization of the network. Thus, even if it is very efficient, it is not well adapted for BCC body-area networks whose topology is highly dynamic, since the nodes are worn by a user, and thus are susceptible to be added or removed at any time, thus impeding almost all attempts to properly synchronize the whole network. Moreover, since devices in this type of network might be worn all day long for only a few transmission period, synchronization is not really useful, and represents a waste of energy.
Other existing protocols use asynchronous sleep schedules. In these protocols, each node wakes-up and sleeps independently from all other nodes. When a transmitting node wants to reach another node, it sends a long preamble, which has a size longer than the maximum time between two wake-up times of any receiving node. Thus this long preamble will “catch” the target receiver and send data afterward. Usually, the long preamble is preceded and immediately followed by contention elements used for detecting concurrent transmissions and therefore avoid collisions. This approach is very powerful since it enables to have an asynchronous network without coordinating node. However the long preamble is a significant overhead. Indeed, as soon as a receiving node catches a long preamble indicating that a transmission will start; it has to wait until the end of the preamble before getting the data, which represents a considerable loss of energy.
A possible solution for reducing the overhead resulting from the long size of the preamble would be to segment it into several micro-preambles separated by listening slots. Thus, it has been proposed some protocols wherein micro-preambles as small as possible are used, in order to decrease the delay cause by the long preamble. In these protocols, the first micro-preamble forming the complete preamble is preceded by a contention element, and the last one is also followed by a contention elements. However, these protocols are not as energy-efficient as it would be expected for body-area networks such as medical sensor networks comprising sensor devices laid on a user's body.